The present invention relates generally to metal sheet products for constructing bodies of motor vehicles, for example, using increased percentage sheet thickness reduction ratios attainable with a work roll texture in a single final stand of a rolling mill over that attainable with directionally ground rolls in multiple stands. The metal sheet provided has enhanced functional properties for subsequent manufacturing processes, such as forming capability in presses, spot weldability, resistance to galling or adhesive metal transfer from the sheet surface to the tool surface in presses that form the sheet into components, and the appearance of sheet surfaces before and after they are painted or coated.
The disclosure of Applicants' earlier U.S. Pat. No. 5,025,547, issued Jun. 25, 1991, is hereby incorporated by reference. This patent concerns itself with micro-engineered surface textures for both work and back-up rolls of each stand of a rolling mill so that each stand is tailored to its particular rolling conditions involving lubrication, wear and traction, with the final stand of the mill reducing the thickness of the strip or sheet in a manner that embosses the surface of the sheet product with the surface texture of the work rolls. This is described at the bottom of column 3 of the patent, beginning with line 63. The surface texture of a work roll in the last stand of the rolling process is shown in the micrograph of FIG. 5 of the patent drawings in U.S. Pat. No. 5,025,547. It should be noted that textures formed in and on a strip surface in all stands but the last one are generally eliminated from the sheet surface by the action of work rolls in later stands of the rolling mill.
In addition, the work rolls of the final stand of Applicants' above patent can be provided with a continuous helical groove, as described at the top of column 4 of the patent. Such a texture would produce the sheet surface shown in FIG. 28 of Applicants' present drawings. The continuous helical groove texture is anisotropic (directional) and the resulting texture on the sheet is also directional, as shown in FIG. 28 of the present application. It is difficult with such a texture to take massive reductions in sheet thickness, i.e., reduction ratios that are at or in excess of 55%. The purpose of the groove texture is to remove lubricant by squeezing the lubricant to the channels in the roll texture. Hence, the sheet/work roll interface is not sufficiently lubricated so as to effect a massive thickness reduction in the final stand of a cold rolling process. The helicoidal groove texture was designed for low reduction rolling processes where the goal is to roll at high speeds to produce a sheet product that is very bright, i.e., has enhanced specular reflectivity.
Also, with the crater texture of FIG. 5 of the above patent, such massive reductions are not contemplated, as an objective of the patent is to produce an isotropic and non-directional surface of the final sheet product by embossing the sheet surface with the work roll surface texture prior to the sheet exiting the rolling mill. Any massive reductions in thickness cause substantial smearing of the sheet surface, as explained in detail hereinafter. In the above patent, the reductions are in the range of 15% to 54%, which range contains sheet thickness reductions that are substantially in excess of those taken during "temper" rolling, which involves sheet thickness reductions on the order of three to ten percent.
Steel and aluminum sheet are products made in rolling mills that employ work rolls to engage the sheet in a thickness reducing process. The ground surfaces of the work rolls can be provided with different textures using such finishing techniques as sand blasting, electric discharge texturing (EDT), CO.sub.2 laser texturing, and electron beam texturing. These methods can provide a variety of micro-roughness morphologies ranging from minute craters, which are positioned in a discrete fashion along the work roll surfaces (such as is shown in FIG. 3 of the present drawings), to rugged irregularities which possess a significant random element (such as shown in FIG. 13). Laser and electron beam technologies are able to provide deterministic crater patterns in work roll surfaces, whereas the crater pattern provided by electric discharge machining has a substantial random element, as shown in FIG. 13. These technologies are generally described in Iron and Steel Engineer, Vol. 68, No. 8, August 1991, and in the Applicants' article "Focused Energy Beam Work Roll Surface Texturing Science and Technology", Journal of Materials Processing and Manufacturing Science, Vol. 2, July 1993.
The work roll surface textures provided by the aforementioned technologies are used to emboss the sheet surface during light percentage sheet thickness reduction rolling processes (e.g. typically equal to or less than 5% thickness reduction). A work roll surface having a crater texture similar to that shown in FIG. 3 will imprint the sheet surface in the manner shown in FIG. 4. This provides the sheet surface with an array of annular recessions surrounding plateau regions, the surfaces of the latter displaying the ground finish on the work roll prior to texturing.
Presently, such textured work roll surfaces are commonly used to manufacture autobody sheet. However, a number of significant rolling process-related problems continue to persist for reasons discussed below. These problems tend to worsen as the percentage sheet thickness reductions taken during a rolling process become heavier and heavier, thereby precluding the use of the texture in accompanying FIG. 3 beyond a certain light reduction.
For example, the percentage sheet thickness reduction ratios achievable with directionally ground rolls bottoms out at relatively low roll gap forces and roll torques, as discussed in detail hereafter.
Similarly, with a small sheet thickness percentage reduction ratio in a cold rolling process (e.g., typically less than 3%) at a relatively low speed (e.g., 500 ft/min) with no lubricant, the topography of a work roll surface may be only partially imprinted or embossed on the sheet surface due to imperfect texture formation on the work roll surface. Such imperfect texture formation can be due to irregular absorption of beam energy by various alloying agents in the work roll steel, for example, thereby leading to an imperfect work roll surface texture. FIG. 5 of the accompanying drawings is an example of imperfect texture formation, FIG. 5 showing a stylus rendered topography of a representative area of a steel sheet surface that shows imperfect embossing of a work roll annular crater texture (similar to that shown in FIG. 3) foraged by electron beam technology.
The embossed sheet roughness typically meets the functional requirements of steel autobody sheet and can have a better painted surface appearance than that of a conventional sheet surface rolled with a work roll having a substantially anisotropic roughness in the form of a directional grind. However, the distinctness of image of the painted surface of a sheet textured with a discrete roughness is not optimized due to the background roughness imparted from the ground portions of the roll surface (as seen in accompanying FIG. 4), as well as by the texture itself which may show through a painted finish.
Wear debris generation during the rolling of metal sheet is one of the most significant rolling process problems associated with roll surface textures produced with the CO.sub.2, electron beam, and electric discharge texturing devices. In order to better understand the nature of this wear mechanism, some mention of the texture morphology produced with these devices is in order. In the case of the CO.sub.2 laser and electron beam devices, a single element of the roughness produced with these devices, known as a "crater," is generated by one or more pulses of sufficient average energy density (which is the product of the beam intensity and the pulse activation time) so as to melt a microscopic quantity of the work roll surface, the individual pulses consisting of electromagnetic radiation in the case of the CO.sub.2 laser, or accelerated electrons in the case of the electron beam device. Each crater consists of a nearly annular rim, which is raised relative to the average roughness of the roll surface, and a recessed region, typically referred to as a depression, formed in the roll surface, as shown in FIG. 8. The crater rim is generally inclined relative to the average roll roughness such that its angle of inclination or slope is quite steep (e.g., 15.degree. to 50.degree.). By adjusting the angle of a high velocity gas assist relative to the position at which the pulsed beam impinges on the roll surface, the CO.sub.2 device can produce an asymmetric crater morphology which consists of a single raised "hump" and a single depression. FIG. 10 is a stylus-rendered topography of such raised humps in a representative area on a work roll surface. The slope of any one hump relative to the average roll roughness is also quite large.
The embossed sheet surface texture resulting from light percentage sheet thickness reduction rolling with a work roll texture provided by electric discharge technology (EDT) typically consists of a series of plateaus and recessed regions, in no discernible order, which are generated (in reverse) on the work roll surface by multiple sparks of discharge electrodes through a dielectric (i.e., initially non-conductive) fluid medium. The plateau edges are also quite steep and jagged. This is seen in FIG. 13 of the accompanying drawings. FIG. 13 is a stylus-rendered topography of a 2008 aluminum alloy sheet surface, the steep and jagged surface being the imprint of a work roll surface textured with an electrical discharge device.
A microcutting wear mechanism, therefore, tends to be the dominant mode of wear debris generation in the roll bite of textured work rolls. With work rolls used in rolling automotive sheet, for example, the average crater rim height or the average height of an asymmetric "hump" typically exceeds the average background work roll roughness which results from the roll grinding operation. As such, microcutting occurs when the crater rims or asymmetric "humps" plow the sheet surface thereby dislodging small particles from the sheet surface. The total volume of wear debris generated in the roll bite when rolling with a textured roll is proportional to the sliding distance between the work roll and the sheet surfaces. The sliding distance is a function of sheet gauge thickness, thickness reduction ratio, and the work roll diameter. Another contributing factor to wear debris generation in the textured roll bite is the average slope of the crater rim or the asymmetric "hump" relative to the average background roll roughness. If this slope generally exceeds 20.degree. or so, then one can expect high levels of wear debris in rolling processes where sheet thickness reductions exceeding 15% are taken.
Excessive wear debris thus leads to an aesthetical problem with the final sheet product since the debris can either be rolled into the sheet, or it can be transferred to the work roll surface where it can act like sharp cutting edges that further damage the sheet surface. For this reason, the use of textured rolls has generally been limited to temper, light percentage sheet thickness reduction ratio rolling, as there is relatively little sliding between the work roll surface and the sheet surface. Such a low reduction tends not to generate a substantial amount of debris.
Temper rolling is occasionally performed in a rolling mill that has a four-high configuration, i.e., two backup rolls and two work rolls. In this case, the contact stresses between the work roll and the backup roll are much higher than those at the interface between the strip and the work roll. The reason for this stems from the fact that the width of the area over which the backup roll and work roll surfaces come into contact is significantly smaller than the width of the area of contact between the work roll and sheet surfaces. Also, the sheet is typically put under a tensile stress which acts to reduce the normal contact stress required to achieve a desired reduction. Hence, backup roll surface wear and ultimately severe damage of the backup roll surface may result from crater rims (in the cases of the laser or electron beam technologies) or sharp cutting edges (in the case of the electric discharge technology) repeatedly indenting the backup roll surface during the rolling process.
Work roll steel is generally harder than the backup roll steel and hence the highest portions of the textured work roll surface may cut into the surface of the backup roll leading to the generation of small steel particles and recessions in the backup roll surface. With some simple engineering estimates, it is possible to demonstrate that, at least in the case of the annular crater morphology in FIG. 8 or asymmetric "hump" morphology in FIG. 10, the work roll texture will indent the backup roll surface under rolling process conditions which are commonly found in the aluminum industry. This first requires an estimate of the lubricant film thickness between the work roll and the backup roll surfaces to determine if the lubricant film separates the two surfaces or if the texture height exceeds the film thickness on the average. Under those conditions in which the work roll surface texture does indeed come into contact with the backup roll surface, it is then necessary to determine whether or not the associated contact stresses along the work roll/backup roll contact are large enough to promote backup roll surface damage. The following developments, while limited to the case of the annular crater morphology, such as shown in FIG. 3, may also be extended to the case of the hump morphology of FIG. 10.
The lubricant film thickness between the work roll and the backup roll can be estimated using the well-known Dowson and Higginson formula for isothermal line contacts using the following expression as found in "Elastohydrodynamic Lubrication" by D. Dowson and G. R. Higginson, Pergamon Press, London, 1966: ##EQU1## where: a.sub.sw =.sqroot.y.sub.i .beta.R.sub.w ; width of sheet/work roll contact (along rolling direction)
u.sub.b, u.sub.w ; backup roll and work roll surface speeds, respectively, relative to contact region PA0 w=.sigma..sub.y (a.sub.sw L.sub.sw); load on the work roll due to sheet deformation per unit length (along the roll axis and hence transverse to rolling direction) PA0 y.sub.i ; initial sheet thickness prior to rolling PA0 E.sub.b ; Young's modulus of backup roll PA0 E.sub.w ; Young's modulus of work roll PA0 E'; effective Young's modulus ##EQU2## ; reciprocal of effective Young's modulus G=.gamma.E'; dimensionless material parameter PA0 L.sub.sw ; unit length of rectangular contact patch (along roll axis and hence transverse to rolling direction) of sheet and work roll interface PA0 L.sub.wb ; unit length of rectangular contact patch (along roll axis and hence transverse to rolling direction) of work roll and backup roll interface PA0 R.sub.b ; radius of backup roll PA0 R.sub.w ; radius of work roll ##EQU3## PA0 .gamma.; pressure viscosity coefficient PA0 .mu..sub.o ; base viscosity of lubricant PA0 .nu.; Poisson's ratio PA0 .sigma..sub.y ; tensile yield strength of sheet PA0 u.sub.w =u.sub.b =7.64 m/s PA0 y.sub.i =0.001 m PA0 E.sub.w =E.sub.b =207 GPa PA0 L.sub.sw =0.0254 m PA0 L.sub.wb =0.0254 m PA0 R.sub.w =0.27 m PA0 R.sub.b =0.64 m PA0 .beta.=0.03 PA0 .gamma.=14.5 GPa.sup.-1 PA0 .mu..sub.o =2.7.times.10.sup.-3 Pa.sec PA0 .nu..sub.w =.nu..sub.b =0.33 PA0 .sigma..sub.y =1.4.times.10.sup.8 Pa (for aluminum alloy 2008) PA0 a.sub.sw =0.0028 m PA0 w=1.0.times.10.sup.4 N PA0 E'=232 GPa PA0 G=3364 PA0 R'=0.19 m PA0 U=4.7.times.10.sup.-13 PA0 W=8.9.times.10.sup.-6
; effective roll radius ##EQU4## ; dimensionless speed ##EQU5## ; dimensionless load .beta.; sheet thickness reduction ratio
The following material and process parameters, which are representative of many four-high rolling configurations in the aluminum industry, are substituted into Equation (1):
where the values of the contact lengths L.sub.sw and L.sub.wb are chosen for the purpose of concept illustration only. Note that the percentage sheet thickness reduction ratio is simple .beta..times.100, or in the present situation, 3%.
The calculated quantities are thus:
where the appropriate unit designation is "Pascal" for "Newton/meter.sup.2 " with the prefix "G" representing giga and which implies multiplication by "10.sup.9."
Using the above values in Equation (1), the lubricant film thickness between the backup roll and work roll given by Equation (1) is EQU h.sub.min =0.43 .mu.m
It is evident that the crater rims carry most of the load since the film thickness h.sub.min is much smaller than a typical crater rim height (e.g., 3.0 .mu.m to 10.0 .mu.m) produced with either the laser or electron beam technologies. Hence, the majority of the craters on the work roll will come into direct contact with the backup roll surface. Since Equation (1) is appropriate for isothermal line contracts, it is likely that the estimate for h.sub.min will be even less due to thermal effects (from plastic heating of the sheet, interfacial friction, etc.) on the lubricant viscosity.
An estimate of the total pressure or normal contact stress carried by the crater rims is thus needed in order to explore the likelihood of work roll surface texture indentation of a backup roll surface. One must first calculate the width a.sub.wb of the rectangular contact area between the work roll and the backup roll surfaces, as shown in FIG. 6. This is accomplished using classical Hertzian contact theory, a discussion of which is found in "Contact Mechanics" by K. L. Johnson, Cambridge University Press, New York, 1985. For two elastic cylinders in contact under a normal load w per unit axial length, the resulting plane contact zone has a width of a.sub.wb along the rolling direction. Hence, ##EQU6## where the numerical values have been taken from the previous list of process parameters and material properties.
In the hexagonal crater pattern shown in FIG. 3 (produced with the electron beam texturing technology), and schematically in FIG. 7, the unit cell is a parallelogram. Within each unit cell lies one complete crater. The area of a parallelogram A.sub.p is given by ##EQU7## where C.sub.s is the center-to-center spacing between adjacent craters in a single cell in the hexagonal pattern as indicated in FIG. 7. Thus, the percentage of area covered by the crater rim in a single unit cell, %A.sub.cr, may be expressed as ##EQU8## where r.sub.o and r.sub.i are the outer radius and inner radius of the crater, respectively. A typical crater produced with the electron beam roll texturing device has an outer radius, r.sub.o, of 76.2 .mu.m, a rim width of 25.4 .mu.m, and hence an inner radius, r.sub.i, of 50.8 .mu.m. A typical center-to-center spacing, C.sub.s, is 203 .mu.m. The area of the crater rim is thus 10.1.times.10.sup.-9 m.sup.2, the area of a unit cell parallelogram, A.sub.p, is 35.6.times.10.sup.-9 m.sup.2 and the percentage of area of a parallelogram cell covered by the crater rim, %A.sub.cr, is 28%. The number of craters "N" within a small rectangular area of length 0.025 m (along the roll axis and hence transverse to the rolling direction) and width a.sub.wb =204.2 .mu.m (as the previously calculated contact width in the rolling direction) along the work roll/backup roll contact region is given by ##EQU9## where N has been rounded to the nearest integer value.
The total area, A.sub.T, covered by all of the crater rims along a unit length is thus ##EQU10##
The average pressure, p.sub.A, distributed across the crater rims along the sheet/work roll interface is therefore ##EQU11##
It is appropriate to assume that the pressure, p.sub.A, is transferred to the work roll/backup roll interface and hence onto those crater rims momentarily at that interface. The yield strength of a backup roll may be as high as 1.0 GPa. Hence, an estimate of the backup roll indentation pressure is 2.6 GPa since the indentation pressure is approximately 2.6 times the yield strength of the backup roll material. Obviously, a 7.4 GPa pressure on the crater rims exceeds the 2.6 GPa indentation pressure of the backup roll surface. Therefore, it is likely that a crater rim will indent the backup roll surface, and the real area of contact will extend beyond the total area covered by the craters even in situations where the percentage sheet thickness reduction ratio is 3%. With time, this will lead to severe damage of the backup roll surface. Such damage can only arrested by changing the backup roll and this leads to production downtime, increased process intensity, added manpower, and increased cost.
Substantial surface wear of a textured work roll occurs when the protruding rims or cutting edges of the work roll surface dislodge from the work roll surface during rolling. In the case of the annular crater morphology formed by the impact of a CO.sub.2 laser beam, for example, the crater rim is formed by the rapid acceleration of a microscopic pool of molten metal onto the that portion of the ground surface of the roll that surrounds the central depression. The metal which solidifies on the roll surface (i.e. the crater rim) attaches itself to the local roll roughness which is typically a ground finish. The strength of this bond is dependent upon the adhesion of the solidified material to the roll surface. This bond may, in fact, be quite weak since the molten metal may not sufficiently "fill" the ground roll roughness. Hence, the crater rim can dislodge from the work roll surface when subjected to the reciprocating contact pressures in the roll bite. This causes a degradation of the textured work roll surface which ultimately requires roll redressing. The problems associated with adhesion of the molten material which solidifies to form the annular crater rim during CO.sub.2 laser texturing of work rolls are discussed in U.S. Pat. No. 4,806,731, dated Feb. 21, 1989 to Bragard et al.
There is no possibility of forming a crater depression without a rim since melted material under the impinging beam flows radially out of a growing depression, with outward fluid velocities of melted material being primarily induced by a surface tension spatial gradient extending along the surface of a molten pool formed by the beam. This gradient is the product of a surface tension variation with temperature and a surface temperature spatial gradient. Variation in the surface tension gradient produces a shear stress which accelerates the surface of the molten material. The shear stress, .sigma., is related to the surface tension spatial gradient through the following relation: ##EQU12## where .zeta.' is the surface tension variation with temperature. The distribution of energy in a beam pulse typically decreases radially outwards from the pulse center and thus the local work roll surface temperature decreases radially outwards from the impingement point of the beam. By this equation, the surface tension thus increases radially outwards from the impingement point. Hence, shear stress on a layer of molten metal is significant enough to displace the melt to the banks of the depression, and the rapidity of the heating process overrides subsurface fluid motion and, hence, more material is carded radially outwards than can be replaced by any recirculating flow beneath outwardly flowing layers of molten work roll surface material. A buildup of material results, followed by rapid freezing of the material, thereby resulting in the formation of an annular rim or lip around the depression. A description of the surface melting and shearing effect during laser and electron beam processing of materials may be found in "Thermocapillary Convection in Materials Processing," by M. M. Chen, in Interdisciplinary Issues in Materials Processing and Manufacturing, S. K. Samanta, R. Komanduri et al eds. ASME Publications, (1987), pages 541-558.
The aforementioned melting and surface shearing process occurs both with and without a gas assist, although the gas assist will modify the maximum height of the molten fluid that is to ultimately form the solidified lip. If the velocity of the gas assist is made to be excessively high, a crater lip is still formed since the aforementioned surface shearing occurs as soon as a molten pool forms under the beam. In addition, a high velocity gas assist will tend to expel molten material from the evolving crater beneath the beam. A fraction of the expelled molten material can re-deposit onto the work roll surface and then solidify into numerous sharp cutting edges. The cutting edges from the re-deposited material will generate wear debris in heavy percentage sheet thickness reduction ratio rolling processes.
Another problem arises from the relative crater spacing along the work roll surface. In the rolling process, the work roll surface moves at a faster speed than the sheet surface prior to the neutral plane, at which the roll and the sheet surfaces have the same speed. After a sheet surface element passes the neutral plane, the surface element moves at a faster speed than the work roll surface. (This phenomena is discussed in detail in Applicants' earlier patent incorporated herein by reference.) If the reduction is generally less than about 5%, then there is minimal relative smearing action between the sheet and the roll surfaces and a nearly perfect imprint of the roll texture onto the sheet surface results. At heavier reductions, the craters in the work roll surface indent the sheet surface and smear the sheet surface toward the rolling direction prior to the time when a sheet surface element reaches the neutral plane; smeared tracks are thus formed on the sheet surface (i.e., forward smearing). After the sheet surface element passes through the neutral plane, the same action is repeated but in the opposite direction since the sheet surface speed exceeds the roll surface speed due to volume conservation of plastic deformation (backward smearing). The net effect of the backwards and forwards smearing action is the formation of short and narrow "tracks" on the sheet surface, and the sheet texture is thus significantly distorted.
FIGS. 9a and 9b of the drawings show the surface morphologies of two sheet surfaces of aluminum alloy 2008 which were rolled with a texture similar to the asymmetric hump texture in FIG. 10 at 35% and 60%, respectively, under otherwise identical rolling process conditions. In each case, the sheet surface textured does not faithfully represent the imprint of the work roll surface texture. This is especially evident in FIG. 9b where the sheet texture displays a substantial directional component due the fact that the humps in the work roll indent and plow the sheet rather than simply indent the sheet as was the case with the light reduction rolling process that produced the sheet surface shown in FIG. 4. Thus, a higher reduction in sheet thickness tends to elongate the crater impressions on the sheet surface, thereby forming long, smeared tracks since the craters generally lie along a helicoidal course on the work roll surface. As the percentage sheet thickness reduction ratio is increased, the tracks due to adjacent craters or humps may connect-up with one another since the average length of a single track due to a single texture element increases with increasing reduction and there is little control over crater placement or overall crater pattern on the roll surface during CO.sub.2 laser texturing where external chopping of a continuous wave beam is involved. This leads to the formation of grooves in the sheet surface which, when taken collectively, form an anisotropic roughness. Hence, it is possible to start with a substantially non-directional texture (such as that shown in FIG. 3) on the work roll surface and end up with a roughness on the rolled sheet surface that has a significant directional component.
These track effects can cause the sheet surface to have a directional appearance such as that found on a ground surface finish or that of the grating texture shown in FIG. 28. Such a surface may in some instances be undesirable from a customer standpoint, especially in situations where the customer desires a quasi-isotropic sheet surface roughness, as the optical properties of such a surface are less dependent upon the direction from which the sheet surface is observed by the human eye and such a surface will tend to retain lubricant rather than freely channeling lubricant during a forming cycle.
In general, it can be concluded that the textures provided by the aforementioned CO.sub.2 laser texturing process cannot be used in heavy reduction (i.e., equal to or greater than 15% thickness reduction) aluminum rolling processes. The reasons are as follows: (1) a high sheet thickness reduction ratio results in a high roll separation force. This enhances the tendency of the crater rims (or humps in the case of the asymmetric morphology shown in FIG. 10) to damage the backup roll surface due to the previously discussed contact stress mechanism, and thereby leads to premature wear of the crater rims; (2) if the annular craters are too close together (i.e. along the circumferential direction on the work roll surface), it is estimated that a 15% sheet thickness reduction may result in the formation of discrete smear tracks which, in situations involving substantially greater thickness reductions than the aforementioned 15%, may interconnect to form continuous tracks in the sheet surface. The interconnected tracks form "rough" bands, while on the surrounding (untextured) sheet surface, which is smeared by relatively flat areas of the roll, there appear smoother and narrower bands from the grinding process. FIG. 11 shows an aluminum alloy 2008 sheet surface rolled at 40% reduction with the annular crater roll surface morphology created by CO.sub.2 laser texturing. The center-to-center crater separation is short enough to cause tracks to form on the sheet surface; (3) since the CO.sub.2 laser beam is mechanically chopped with a serrated disk, it is not possible to precisely control the position of one crater relative to its neighbors, i.e., to produce hexagonal or square-shaped cells. This is currently only possible with either the electron beam technology, since this technology has been adapted from the rotogravure printing with an internally pulsed CO.sub.2 wherein the active elements in the laser resonator are modulated with a radio frequency device. Additional details oil the application of electron beam technology to rotogravure printing may be found in "A Rapid Electron Beam Engraving Process for Engraving Metal Cylinders" by W. Boppel, Optik, Vol. 77, No. 2, (1987), pages 83-92; (4) the craters lips or humps produced with the CO.sub.2 laser texturing process will lead to prohibitively high levels of wear debris generation during large sheet thickness reduction rolling processes. Excessive wear debris puts an added burden on the rolling mill oil filtration house and will ultimately lead to termination of the rolling process. Similar concerns can be raised about the electron beam textures.
The painted appearance of sheet material rolled with laser textured rolls is often objectionable to sheet customers in the automotive industry. The annular crater roll texture leads to an annular recession in the sheet surface, and the hump texture leads to a nearly circular depression in the sheet surface. In either case, the sheet surface depressions are surrounded by flat areas which serve as bearing areas through which the load from a forming tool is transferred to the sheet. The strains in pressworking operations may not be large enough to cause the depressions on the sheet to completely disappear from the sheet surface. The embossed sheet texture, therefore, may show through the painted finish giving the paint finish a background texture. This is often a basis for rejection of the formed sheet component, especially for luxury class automobiles. Discussion of a texture remnant after pressworking is found in Chapter 5 of "Optimierung der Oberflachenmikrogeometrie von Aluminiumfeinblech fur das Karosserieziehen" (Springer-Verlag, 1988). In English, the title is "Optimizing the Surface Microgeometry of Aluminum Sheet for Automotive Body Panel Drawing" by R. Balbach.